# where 筛选行
# select * from 表 where 条件
    # 范围查询
        # > < >= <= = !=/<>
        # between a and b
        # in (1,2,3,4)   n选1
    # 模糊查询
        # like
            # % 一个百分号代表任意长度的任意字符
                # 'a%'
                # '%ing'
                # '%a%'
            # _ 一个下划线代表一个任意字符
                # 'a__'
        # regexp
            # '^a'
            # '\d+'
    # is is not
        # is null
        # is not null
    # 逻辑运算
        # and
        # or
        # not